Pushbutton-operated overload circuit breaker

ABSTRACT

This invention relates to a pushbutton overload circuit breaker for protecting electrical components, such as semiconductor devices, which are sensitive to excess currents. In particular the invention provides a power transistor in parallel with a release device of the circuit breaker and electrical control means rendering the power transistor nonconductive when the input current exceeds a predetermined value. The electrical control means may include a control resistor and a semiconductor device connected to the base of the power transistor. In normal use of the circuit breaker a major portion of the input current will flow through the power transistor with a minor portion flowing through release device. However, when the input current exceeds the predetermined value the electrical control means will render the power transistor inoperative thus allowing all the input current to flow through the release device which trips the circuit breaker.

United States Patent 2,955,237 10/1960 Wyndham Inventor Jakob Ellenbcrger Altdorf, near, Nurnberg, Germany Appl. No. 818,620 Filed Apr. 23, 1969 Patented Jan. 26,1971 Assignee Ellenberger & Poensgen G.m.b.H.

Altdorf near Nurnberg, a firm of Germany PUSHBUTTON-OPERATED OVERLOAD CIRCUIT BREAKER 17 Claims, 4 Drawing Figs.

3,409,803 11/1968 Dewitt, Jr.

ABSTRACT: This invention relates to a pushbutton overload circuit breaker for protecting electrical components, such as semiconductor devices, which are sensitive to excess currents. In particular the invention provides a power transistor in parallel with a release device of the circuit breaker and electrical control means rendering the power transistor non-conductive when the input current exceeds a predetermined value. The electrical control means may include a control resistor and a semiconductor device connected to the base of the power transistor. In normal use of the circuit breaker a major portion of the input current will flow through the power transistor with a minor portion flowing through release device. However, when the input current exceeds the predetermined value the electrical control means will render the power transistor inoperative thus allowing all the input current to flow through the release device which trips the circuit breaker.

PUSIIBUTTON-OPERATED OVERLOAD CIRCUIT BREAKER The invention relates to a pushbutton-operated overload circuit breaker for protecting electric components, such as semiconductor devices, which are sensitive to excess current.

US. Pat. specification No. 3,307,122 discloses a pushbutton overload circuit breaker having a rapid electromagnetic release. The circuit-breaking time of this known overload breaker is 4 to 6 msec with maximum energization. Since no resistance change occurs in this overload circuit breaker up to the instant when the contacts are opened when there is excess current, the rise in the circuit to be protected depends upon the total impedance. The impedance of the circuit to be protected generally consists of the line impedances and the resistances of the semiconductor components to be protected and of the current source, of which the inductance is generally low. Consequently, the excess current reaches its full value within a very short time of less than 1 msec, this maximum value being determined almost exclusively by the ohmic resistance of the circuit. Since the circuit-opening time is about 4 to 6 msec in the known overload circuit breaker, circuits comprising semiconductor components cannot be protected against excess current by such known overload circuit breakers. This applies both to direct currents and to alternating currents, since in the case of alternating currents, with a circuit-breaking time of about 5 msec, the full peak value of the excess current is reached within a quarter-cycle in the case of 50-cycle alternating current.

In the protection of semiconductor components, it is necessary that the circuit-breaking should take place, on occurrence of an excess current, within a period in which the boundary load value I2dt of the semiconductor crystal is not exceeded.

According to the present invention there is provided a pushbutton operated overload circuit breaker-for protecting electrical components which are sensitive to excess currents, such circuit breaker comprising a switching contact for switching the circuit on and off, a release device connected in series with the switching contact, the release device being operable when the current therethrough exceeds a predetermined value to actuate the switching contact and render the circuit inoperative, a power transistor having its emitter and collector connected in parallel with the release device and electrical control means connected for rendering the power transistor nonconductive when the input current to the circuit exceeds a predetermined value. The electrical control means may include a control resistor which may be in either series or parallel with the release device. Further, the electrical control means may include a semiconductor device in the form of a diode of a control transistor. Such a semiconductor device is connected to the base of the power transistor and to one of the input terminals. Where the semiconductor device is in the form of a diode the control resistor must be of such value that the voltage drop across it is lower at rated current than the punch-through voltage of the diode and of the power transistor.

When the input current increases, the voltage drop across the control resistor also increases, so that the base voltage of the power transistor becomes greater than the voltage at the emitter. The power transistor is thus rendered nonconductive so that the .current now flows through the release device thereby causing the circuit breaker to open the circuit in its normal switching time.

If the overload switch includes a conventional electromagnetic release and a conventional thermal release, it is possible for the heating winding of the bimetallic strip of the thermal release to constitute the control resistor and for the magnetic coil to be connected between the emitter and the collector of the power transistor. It is furthermore possible in an overload circuit breaker having thermal release for the bimetal strip itself to form the control resistor and for its heating winding to lie between the emitter and the collector of the power transistor. In a modification two heating windings may be provided, one of which constitutes the control resistor and the other of which is connected to the collector and to the emitter of the power transistor.

The power transistor may be either a PNP transistor or an npn transistor. When an npn power transistor and a control transistor is employed the base of the control transistor is connected to the emitter of the power transistor through a variable resistor.

The electronic circuit may be mounted on a circuit plate which preferably consists of a metal having good thermal conductivity and which rapidly dissipates heat away from the power transistor. For the electrical connection of the electronic circuit, the circuit plate is provided with plug-in contacts for plugging into sockets in a terminal board, into which the pushbutton-operated overload circuit breaker is also plugged by means of its plug-in contacts.

In the circuit breaker described, a relatively high control power must be provided, because the control takes place not only in dependence upon current but also in dependence upon voltage. In order that the circuit breaker may be operated with lower control power, and only in dependence upon current, the connection of the base of the power transistor to the other of the input terminals may be omitted.

In normal use of the present circuit breaker a major portion of the input current flows through the power transistor with the minor portion flowing through the release device. However, when the input current exceeds a predetermined level, the electrical control means will render the power transistor nonconductive so that all of the input current will now flow through the release device. When this occurs, the release device will actuate switching contacts in the circuit breaker to render the circuit inoperative.

Since small control transistors generally exhibit shorter switching delay than large power transistors, it is necessary for the control transistor for closing the whole circuit to remain nonconductive for a certain time in order that the power transistor may be turned on. In a further development of the invention, a capacitor is for this purpose connected between the base of the control transistor and the connecting conductor between the control resistor and the emitter of the control transistor.

This capacitor may also be employed to eliminate the response sensitivity at the circuit-closing instant when circuitclosing current peaks occur at circuit-closing in the apparatus to be protected. For this purpose, there is provided between the control resistor and the base of the control transistor a series resistor through which the capacitor is charged.

In order that high excess currents may be reliably interrupted even at high rated currents, there is provided between the power transistor and the control transistor a second control transistor whose base is connected to the collector of the first control transistor, whose emitter is connected to the base of the power transistor and whose collector is preferably connected through a series resistor to the corresponding end of the magnet coil.

Illustrative embodiments of the invention will npw be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates one form of circuit breaker;

FIG. 2 illustrates another form of overload circuit breaker;

FIG. 3 illustrates the overload circuit breaker with a similar electronic circuit to the embodiment of FIG. 2, but employing an npn power transistor instead of a PNP power transistor; and

FIG. 4 illustrates yet a further form of overload circuit breaker.

The circuit arrangements according to FIGS. 1 to 3 constitute four-terminal networks whose input terminals 1 and 2 are connected to a direct-current source and whose output terminals 3 and 4 are connected to a load (not shown). The upper terminal is connected via a conductor 5 to switching contacts 6 of a pushbutton-operated overload switch 7 (constructed as described in US. Pat. specification No. 3,307,122) to a control resistor 8 and to a magnetic coil 9 of the overload switch 7. The ends of the coil 9 are electrically connected to the emitter and to the collector of a power transistor 10 whose base is connected through a resistor 11 to a conductor 12 which is in turn connected to the lower terminal. The end of the control resistor 8 which is further from the coil 9 is electrically connected to the base of the power transistor 10 through a diode 13 which may be in the form of a Zener diode.

It will be assumed that the load is a transistor circuit whose boundaryload value is 50 A msec. The overload switch employed in this case has a switching time of msec, so that the maximum short circuit current at the terminal connections 3,

4 must be limited to 5 =22.3 A, or

rent source has a unidirectional voltage of 24 volts and the rated load current is 8 amp. The current limitation is effected when the sum of the resistance values of the control resistor 8 and of the coil 9 is 1 ohm. The control resistor 8 must be of such value that at rated load the voltage drop is lower than the punch-through voltage of the diode 13 and of the emitter-base path of the power transistor 10. During normal operation, the transistor carries a major portion of the input current while the coil 9 carries the minor portion thereof. Such a power transistor has in the on state, for a continuous current of about 8 to 10 A, an emitter-collector voltage of about 0.25 to 0.3 V, corresponding to a forward resistance of about 0.04 ohm. It follows from this that the greater part of the continuous'current is taken up by the power transistor if the resistance of the coil 9 is suitably adjusted.

If the input current increases, the voltage drop across the control resistor 8 also increases, whereby the base voltage of the power transistor 10 increases relative to the emitter voltage. The power transistor is thus rendered non conductive, so that the input flows through the magnet coil 9. The overload switch 7 therefore responds, and the circuit is electrically opened by the switching contacts 6. The response speed of the circuit shown in FIG. 1 until the instant of the current reduction depends upon the rate or rise of the current. With a slow rate of rise, the power transistor 10 is correspondingly slowly rendered nonconductive. The current-limiting effect, however, is produced even with a rapid current rise, which may last less than I usec. The circuit arrangement shown in FIG. 1 is intended mainly for load circuits having rapidly rising excess currents.

In the circuit arrangement shown in FIG. 2, there is provided, instead of the diode 13, a transistor 14 whose emitter is electrically connected to one end of the control resistor 8 and whose collector is electrically connected to the base of the power transistor 10. The base of the transistor 14 is connected through a fixed resistor 15 and a variable resistor 16 to the col- 'lector of the power transistor 10. Here again, the power transistor 10 is brought into the conductive state through the resistor 11 in normal operation. The emitter-base voltage of the transistor 14 is therefore very low, so that this transistor 14 is not turned on. As the input current rises, the voltage drop across the control resistor 8 also becomes greater, so that the control voltage for'the transistor 14 in turn becomes greater.

Owing to the fact that the transistor 14 is rendered conductive, the base voltage of the power transistor 10 changes towards positive, so that this power transistor 10 is rendered less conductive and the emitter-base voltage at the transistor 14 increases further. As a result of this voltage rise, the transistor 14 is rendered even more conductive, so that the power transistor 10 is turned off in avalanche manner. The limitation of the load current is effected by the resistance value of the magnet coil 9. As soon as the corresponding current is reached, the tripping takes place.

, The circuit arrangement shown in FIG. 3 differs from that 'shown in FIG. 2 only in that, in the latter, a PNP power transistor l0-is employed, while the circuit arrangement shown in FIG. 3 comprises an npn power transistor 17. In the circuit arrangements shown in FIGS. 2 and 3, the response current may be adjusted by the variable resistor 16 within predetermined limits. The circuit arrangement shown in FIG.

1 can be tuned to the corresponding currents substantially only by the values of the resistors 8, 11 and of the diode 13.

Instead of an overload switch as described in U.S. Pat. specification No. 3,307,122, an overload switch as described in U.S. Pat. specification No. 3,268,688 may be employed. Such an overload switch comprises a thermal release with a bimetal strip provided with a heating winding. In this case, the heating winding of the bimetal strip may be employed as control resistor 8. If, in this case, the response value for the current reduction is adjusted to a correspondingly high value in relation to the rated current, it is possible to protect the intermediate region in an overcurrent characteristic curve which is determined by the heating of the bimetallic strip by the control resistor 8. This step is particularly advantageous when circuit-closing peaks or recurrent current peaks in a circuit arrangement only reach a harmless level.

The electronic circuit may be mounted on an additional circuit plate which may be provided with corresponding plug connections for a simple electrical connection to a terminal board. It is thereby possible for the circuit plate to be simultaneously connected to the associated pushbutton-operated overload switch, which also comprises plug-in contacts, by

. plugging it into the corresponding sockets in the terminal board, and thus to be mechanically secured thereto. In this for the terminal board to be made of a metal having good thermal conductivity in order that it may rapidly dissipate the heat evolved in the power transistor.

Again in the circuit arrangement shown in FIG. 4, the input terminals 1 and 2 are connected to a direct-current source (not shown). Connected in series with the switching contacts 6 and the magnetic coil 9 of the pushbuttonoperated overload switch 7 is a load 18. Instead of the magnetic coil 9, there may be employed the heating winding of a bimetal strip of the thermal release of the pushbutton-operated overload switch 7. A resistor 23 and a potentiometer 24 in series therewith are connected to the ends 19 and 20 of the coil 9 through connecting conductors 21 and 22. The fixed resistor 23 and the potentiometer 24 form a control resistor. The tap 25 is connected to the base of a control transistor 27 through a series resistor 26. A capacitor 28 is situated between the base of the control transistor 27 and the connecting conductor 21. The emitter of the control transistor 27 is connected to the connecting conductor 21, while its collector is connected on the one hand to the base of a second control transistor 29 and on the other hand to a series resistor 30 which is connected at its other end to the connecting conductor 22. The collector of the second control transistor 29 is connected to the connecting conductor 22 through a series resistor 31. The emitter of the second control transistor 29 is connected to the base of the power transistor 10, whose emitter is connected to the connecting conductor 21 and whose collector is connected to the connecting conductor 22 through a series resistor 32.

The overload switch as shown in FIG. 4 operates as follows:

When the overload switch is closed, the second control transistor 29 is rendered conductive through the series resistor 30 acting as a base resistor, so that the power transistor 10 is also rendered conductive. Consequently, the load current flows in the coil 9 and in the emitter-collector path of the power transistor 10, as well as in the series resistor 32, in a ratio determined by their conductivities. The voltage drop across the series resistor 32 must be of such value that the control current determined by the second control transistor 29 and its series resistor 31 and the control current determined by the series resistor 30 for the control transistor 29 ensure j that the control transistor 29 and the power transistor10 are .age drop across the series resistor 30, which is determined by .the through flowing base current of the control transistor 29.

The series resistor 30 must be so chosen that, in the operated state, it carries a relatively low current as compared with the magnet coil 9, so that the current is determined mainly by the resistance of the magnet coil 9 or of a corresponding heating winding.

When an excess current occurs, the voltage drop across the series resistors 30, 31 and 32 increases in proportion with the current increase. The voltage drop across the resistor 23 and the potentiometer 24 also increases, and thus also the voltage at the base of the control transistor 27, so that the latter is rendered conductive and renders nonconductive the control transistor 29 and the power transistor l0,whereby the voltage drop in the protective circuit is increased and in turn causes the control transistor 27 to become conductive in avalanche manner. As a result of this changeover of the control circuit, the load circuit is rendered nonconductive or reduced to the current determined by the resistance of the magnet coil 9 and the series resistor 30 and the resistance of the control transistor 27. This voltage increase at the magnet coil 9, and the consequent current increase, results in tripping of the overload switch and opening of the switching contacts 6.

When the whole circuit is closed, the control transistor 27 is rendered nonconductive for a particular time by the capacitor 28, so that the control transistor 29 and the power transistor can be rendered conductive. The capacitance of the capacitor 28 must be at least of such value that the control transistor 27 is rendered nonconductive over the natural time of the power transistor 10 on circuit-closing at currents below the response value when the tap 25 of the potentiometer 24 is at its highest setting.

The capacitor 28 and, in combination therewith, the series resistor 26 serve to eliminate the response sensitivity when-the whole circuit is closed. The period for which the control transistor 27 is then rendered'non conductive by the capacitor 28 is determined by the time constant resulting from the value of the capacitor 28 and of the control current flowing through the series resistor 26. The control current depends upon the voltage drop in the control system and is thus dependent upon current. The value of the capacitor 28 is thus determined by the height of the circuit-closing current. peak and its duration. At circuit-closing, the capacitor 28 is charged to a value determined by the position of the tap 25 and by the continuous current. If current peaks or current rises thereafter occur,'the voltage difference which must exist at the capacitor 28 in order that the response point may be reachedis lower than at circuit closing, so that in. continuous operation the response sensitivity in turn becomes greater and brief current peaks slightly above the response value can be interrupted. A wide adaption to the most varied conditions of application is possible by tuning of the capacitor 28,in combination with the resistors 23, 24, 26.

The overload switch illustrated in FIG. 4 is intended above all for interrupting high rated currents. With lower rated currents, the second control transistor 29 and the series resistor V 31 may be omitted, so that the base of the power transistor 10 is then connected to the series resistor 30 and to the collector of the control transistor 27.

Of course, npn transistors may be employed instead of PNP transistors.

I claim: i

l. A 'pushbutton operated overload circuit breaker for protecting electrical components which are sensitive to excess currents, such circuit breaker comprising a switching contact for switching the circuit on and off, a release device, said release device being connected in series with the said switching contact and being operable when the current therethrough exceeds a predetermined value to actuate the said switching contact and render the circuit inoperative, a power transistor, said power transistor having its emitter and collector connected in parallel with said release device, and electrical control means, said electrical control'means rendering the power transistor nonconductive when the input current to the circuit exceeds a predetermined value.

2. A circuit breaker according to claim 1 in which the said electrical control means includes a control resistor arrangement, the said control resistor arrangement being adapted to provide a predetermined voltage drop when said input current exceeds said predetermined value.

3. A circuit breaker according to claim 2 which includes an electromagnetic release device, said electromagnetic device having a magnetic coil and a thermal release device, said thermal release having a bimetal strip and a heating winding for heating the bimetal strip, the said heating winding constituting the said control resistor arrangement with the said magnetic coil of the said electromagnetic release device being connected in parallel with the said collector and the said emitter of the said power transistor.

4. A circuit breaker according to claim 2 which includes a thermal release, said thermal release having a bimetal strip and a heating winding for heating the bimetal strip, the said bimetal strip constituting the said control resistor arrangement and with the said heating winding being connected in parallel with the said emitter and collector of the said power transistor.

5. A circuit breaker according to claim 2 which includes a thermal release, said thermal release having a bimetal strip and first and second heating windings, one of said first and,

second heating windings constitutes the said control resistor arrangement. Y

6. A circuit breaker according to claim 1 in which the electrical control means includes a control resistor arrangement and a semiconductor device, said semiconductor device being so connected to the said base of the said power transistor that the semiconductor device is rendered nonconductive upon occurrence of the said predetermined voltage across the said control resistor arrangement. I

7. A circuit breaker according to claim 6 in which the said semiconductor device is a diode, and the value of the said control resistor arrangement is such that the voltage drop across the said control resistor arranged at the rated current is lower than the punch-through voltage of the said diode and the said power transistor.

8. A circuit breaker according to claim 6 in which the said semiconductoris a control transistor, said control transistor having its collector and emitter providing an electrical connection to the base of the said power transistor, the base of the said control transistor being so connected to the said electrical control means that a control voltage is applied thereto as a function of the voltage drop across the said control resistor arrangement.

9. A circuit breaker according to claim 6 in which the said semiconductor is a control transistor, 'withthe said power.

' power transistor being of the npn type and in which the base of the said control transistor is connected via a variable resistor to the emitter of the said power transistor.

11. A circuit breaker according to claim 1 which includes a pair of current input terminals and a resistor, the base of the said power transistor being connected to one of the input terminals via said resistor and the said switching contact being connected to the other of the said input terminals.

12. A circuit breaker according to claim 1 which includes a first control transistor and a second control transistor, the said first control transistor having its base connected to the collector of the said'second transistor, its emitter connected to the base of the said power transistor and its collector connected to the said release device.

13. A circuit breaker according to claim 12 in which the said control means includes a control resistor arrangement, said control resistor arrangement being in the form of a fixed value resistor and a variable resistor, the said fixed value resistor and said variable resistor being connected to the base of 14. A circuit breaker according to claim 13 which includes a resistor, the emitter and collector of the said second control 7 transistor being in series with the said resistor, with the said control resistor and the emitter and collector of the said first control transistor being in parallel with the said release device.

15. A circuit breaker according to claim 12 which includes a capacitor, the said capacitor being connected between the base of the said second control transistor, the said control resistor arrangement and the emitter of the said second control 

1. A pushbutton operated overload circuit breaker for protecting electrical components which are sensitive to excess currents, such circuit breaker comprising a switching contact for switching the circuit on and off, a release device, said release device being connected in series with the said switching contact and being operable when the current therethrough exceeds a predetermined value to actuate the said switching contact and render the circuit inoperative, a power transistor, said power transistor having its emitter and collector connected in parallel with said release device, and electrical control means, said electrical control means rendering the power transistor nonconductive when the input current to the circuit exceeds a predetermined value.
 2. A circuit breaker according to claim 1 in which the said electrical control means includes a control resistor arrangement, the said control resistor arrangement being adapted to provide a predetermined voltage drop when said input current exceeds said predetermined value.
 3. A circuit breaker according to claim 2 which includes an electromagnetic release device, said electromagnetic device having a magnetic coil and a thermal release device, said thermal release having a bimetal strip and a heating winding for heating the bimetal strip, the said heating winding constituting the said control resistor arrangement with the said magnetic coil of the said electromagnetic release device being connected in parallel with the said collector and the said emitter of the said power transistor.
 4. A circuit breaker according to claim 2 which includes a thermal release, said thermal release having a bimetal strip and a heating winding for heating the bimetal strip, the said bimetal strip constituting the said control resistor arrangement and with the said heating winding being connected in parallel with the said emitter and collector of the said power transistor.
 5. A circuit breaker according to claim 2 which includes a thermal release, said thermal release having a bimetal strip and first and second heating windings, one of said first and second heating windings constitutes the said control resistor arrangement.
 6. A circuit breaker according to claim 1 in which the electrical control means includes a control resistor arrangement and a semiconductor device, said semiconductor device being so connected to the said base of the said power transistor that the semiconductor device is rendered nonconductive upon occurrence of the said predetermined voltage across the said control resistor arrangement.
 7. A circuit breaker according to claim 6 in which the said semiconductor device is a diode, and the value of the said control resistor arrangement is such that the voltage drop across the said control resistor arranged at the rated current is lower than the punch-through voltage of the said diode and the said power transistor.
 8. A circuit breaker according to claim 6 in which the said semiconductor is a control transistor, said control transistor having its collector and emitter providing an electrical connection to the base of the said power transistor, the base of the said control transistor being so connected to the said electrical control means that a control voltage is applied thereto as a function of the voltage drop across the said control resistor arrangement.
 9. A circuit breaker according to claim 6 in which the said semiconductor is a control transistor, with the said power transistor being of the PNP type, and in which the said base of the said control transistor is connected via a variable resistor to the collector of the said power transistor.
 10. A circuit breaker according to claim 6 in which the semiconductor device is a control transistor, with the said power transistor being of the npn type and in which the base of the said control transistor is connected via a variable resistor to the emitter of the said power transistor.
 11. A circuit breaker according to claim 1 which includes a pair of current input terminals and a resistor, the base of the said power transistor being connected to one of the input terminals via said resistor and the said switching contact being connected to the other of the said input terminals.
 12. A circuit breaker according to claim 1 which includes a first control transistor and a second control transistor, the said first control transistor having its base connected to the collector of the said second transistor, its emitter connected to the base of the said power transistor and its collector connected to the said release device.
 13. A circuit breaker according to claim 12 in which the said control means includes a control resistor arrangement, said control resistor arrangement being in the form of a fixed value resistor and a variable resistor, the said fixed value resistor and said variable resistor being connected to the base of the said power transistor through the collector and emitter of the said first control transistor.
 14. A circuit breaker according to claim 13 which includes a resistor, the emitter and collector of the said second control transistor being in series with the said resistor, with the said control resistor and the emitter and collector of the said first control transistor being in parallel with the said release device.
 15. A circuit breaker according to claim 12 which includes a capacitor, the said capacitor being connected between the base of the said second control transistor, the said control resistor arrangement and the emitter of the said second control transistor.
 16. A circuit breaker according to claim 12 which includes a resistor, said resistor being in series with the base of the said second transistor and the said control resistor arrangement.
 17. A circuit breaker according to claim 1 which includes a circuit plate of good thermal conductivity, the said switching contact, thE said release device and the said electrical control means being mounted on said circuit plate. 